Purpose: Global DNA hydroxymethylation is markedly decreased in human cancers, including hepatocellular carcinoma, which is associated with chronic alcohol consumption and aging. Because gene-specific changes in hydroxymethylcytosine may affect gene transcription, giving rise to a carcinogenic environment, we determined genome-wide site-specific changes in hepatic hydroxymethylcytosine that are associated with chronic alcohol consumption and aging.
Methods: Young (4 months) and old (18 months) male C57Bl/6 mice were fed either an ethanol-containing Lieber-DeCarli liquid diet or an isocaloric control diet for 5 weeks. Genomic and gene-specific hydroxymethylcytosine patterns were determined through hydroxymethyl DNA immunoprecipitation array in hepatic DNA.
Results: Hydroxymethylcytosine patterns were more perturbed by alcohol consumption in young mice than in old mice (431 differentially hydroxymethylated regions, DhMRs, in young vs 189 DhMRs in old). A CpG island ~2.5 kb upstream of the glucocorticoid receptor gene, Nr3c1, had increased hydroxymethylation as well as increased mRNA expression (p = 0.015) in young mice fed alcohol relative to the control group. Aging alone also altered hydroxymethylcytosine patterns, with 331 DhMRs, but alcohol attenuated this effect. Aging was associated with a decrease in hydroxymethylcytosine ~1 kb upstream of the leptin receptor gene, Lepr, and decreased transcription of this gene (p = 0.029). Nr3c1 and Lepr are both involved in hepatic lipid homeostasis and hepatosteatosis, which may create a carcinogenic environment.
Conclusions: These results suggest that the location of hydroxymethylcytosine in the genome is site specific and not random, and that changes in hydroxymethylation may play a role in the liver's response to aging and alcohol.
Keywords: Aging; Alcohol; DNA hydroxymethylation; Liver; hmeDIP-Chip.